252 SCIENTIFIC AMUSEMENTS. 



much narrower than the other. If quicksilver be poured in it 

 will be found that the mercury will stand highest in the wider 

 limb ; whilst the opposite will be noticed if alcohol or water be 

 used (fig. 104). In the first case the surface of the quicksilver 

 will be convex upwards in both limbs ; whilst in the second both 

 water surfaces will be concave. 



Expt. 297. Apparent Attraction and Repulsion between 

 Floating Bodies. A curious result follows from the nature of the 

 curved surface of the water or other fluid in which a solid body 

 floats ; if the fluid wets the body, as with a piece of cork floating 

 on water or a glass rod suspended therein, the curved surface is 

 concave ;. and, .in consequence, two such floating or freely sus- 

 pended objects placed close together will tend to approach one 

 another and will apparently attract one another (fig. 100). 

 The same thing will be observed if the two floating objects are 

 not wetted by the water, e.g., two pellets of wax or two suspended 

 well-greased glass rods, in which case the curved surfaces of the 

 water are convex (fig. 101). But if one object be wetted by the 

 water and the other not, as when a piece of cork and a lump of 

 wax are used together, or a clean glass rod and a greased one, the 

 curvatures of the water surfaces surrounding the two are not 

 alike, one being concave and the other convex ; in this case the 

 two objects will not approach one another, but if floated on the 

 water close together will separate, apparently 

 repelling each other (fig. 105). 



The floating of a comparatively heavy 

 small object, such as a sewing needle, on water 

 (Expt. 279) is partly due to non-wetting of 

 the polished steel by the water ; a well- 

 oiled small fragment of stone, or other greasy 

 heavy substance not too large, will often 

 behave in the same way. But if the needle 

 be washed with alcohol, or the greasy stone 

 with solution of caustic potash, and placed 

 on the surface of a basin of water whilst 

 still wet, it will immediately sink, because 

 the fluid is now enabled to wet its surface, 

 and consequently no air cushion, so to 

 105. Greased arid speak, is formed between it and the water 



Wat n er glaSS *"** "* buo y in g it} U P Water beetles and man y 

 other insects skim over the surface of a pond 



without sinking into it, because their feet are not wetted by the 

 water, and, consequently, depressions are produced which buoy 

 them up like the needle. 



